Method of manufacturing sintered body

ABSTRACT

A method of manufacturing a sintered body includes a first process of forming a metal powder containing carbon into a compact having a predetermined shape and a second process of baking the compact in a hermetically sealed space so as to produce a sintered body. The hermetically sealed space has an atmosphere having a pressure of 60 kPa to 140 kPa and contains a hydrogen gas and an oxygen gas. The sum of partial pressures of the hydrogen gas and the oxygen gas is not more than 3 Pa.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priorities to Japanese Patent Application No. 2006-102560 filed on Apr. 3, 2006 which is hereby expressly incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sintered body and a method of manufacturing a sintered body.

2. Description of the Related Art

For example, a metal product is manufactured by sintering a compact containing a metal powder in the following manner. A metal powder and an organic binder are mixed and kneaded. The kneaded compound is formed into a predetermined shape, so that a primary compact is produced. Then the primary compact is degreased (debound) so as to remove the organic binder from the primary compact. Thus, a secondary compact (degreased compact) is produced. Thereafter, the secondary compact is baked so as to produce a sintered body.

The secondary compact is baked in a baking furnace under a reduced-pressure (vacuum) atmosphere having a high vacuum of 13 Pa (0.1 Torr) or less, a non-oxidizing atmosphere (see Japanese laid-open patent publication No. 7-224348), or an air atmosphere.

Meanwhile, in the field of powder metallurgy, mechanical characteristics of a resultant sintered body have been improved by properly adjusting the composition of a metal powder. Specifically, it can be seen that mechanical characteristics such as tensile strength and hardness are improved in a sintered body formed of stainless steel powder having a given carbon (C) content (e.g., about 0.8 atm % to about 1.2 atm %).

However, when a secondary compact of a metal powder containing carbon is to be baked by the aforementioned method disclosed in Japanese laid-open patent publication No. 7-224348, the following problems arise.

First, when a secondary compact is baked in a non-oxidizing atmosphere, a hydrogen gas contained in the atmosphere reacts with carbon in the secondary compact. Accordingly, carbon is problematically desorbed from the secondary compact. Furthermore, when a secondary compact is baked in an air atmosphere, an oxygen gas contained in the atmosphere reacts with carbon in the secondary compact. Accordingly, the same problem arises. If such a problem arises, then the carbon content of a sintered body is lowered so as to cause deterioration of mechanical characteristics.

Additionally, because a decrease of the carbon content proceeds when the secondary compact is brought into contact with a hydrogen gas and an oxygen gas in the atmosphere, it is likely to depend upon a shape of the secondary compact. That is, the carbon content tends to be lowered in a secondary compact having a complicated shape with a large surface area. Accordingly, the degree of the decrease of the carbon content varies depending upon the shape of a secondary compact.

Second, the aforementioned high vacuum of 13 Pa (0.1 Torr) or less may cause pressure drop in the baking furnace and production of an oxygen gas from a material of components forming a baking furnace depending upon the kind of the material. Therefore, the same problem as described above may also arise. Furthermore, in order to maintain a high vacuum, it is necessary to provide a special pressure proof mechanism or an expensive pump for a high vacuum in the baking furnace. Accordingly, cost of the baking process is problematically increased.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above drawbacks. It is, therefore, a first object of the present invention to provide a method of manufacturing a sintered body which can efficiently manufacture a sintered body having a desired carbon content at a low cost irrespective of a shape of the sintered body.

A second object of the present invention is to provide a sintered body manufactured by such a method.

According to a first aspect of the present invention, there is provided a method of manufacturing a sintered body. This method includes forming a metal powder containing carbon into a compact having a predetermined shape and baking the compact in a hermetically sealed space so as to produce a sintered body. The hermetically sealed space has an atmosphere having a pressure of 60 kPa to 140 kPa and contains a hydrogen gas and an oxygen gas. The sum of partial pressures of the hydrogen gas and the oxygen gas is not more than 3 Pa.

With the above method, it is possible to efficiently produce a sintered body having a desired carbon content at a low cost.

It is desirable that the atmosphere of the hermetically sealed space primarily should contain an inert gas. It is also desirable to use an argon gas as the inert gas.

Furthermore, it is desirable that the metal powder should have an average particle diameter of 3 μm to 30 μm. It is also desirable that the metal powder should have a carbon content of 0.05 atm % to 2 atm %. Additionally, it is desirable to make the metal powder of an Fe-based alloy material.

The metal powder forming process may include forming a composition containing the metal powder and a binder into a predetermined shape so as to produce a primary compact and removing the binder from the primary compact so as to produce a secondary compact as the compact. It is desirable that the forming process of the composition should comprise metal injection molding of the composition to produce the primary compact.

According to a second aspect of the present invention, there is provided a sintered body manufactured by the above method. This sintered body can have excellent mechanical characteristics.

The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing a method of manufacturing a sintered body according to an embodiment of the present invention; and

FIG. 2 is a vertical cross-sectional view schematically showing a baking furnace (hermetically sealed vessel) used in a method of manufacturing a sintered body according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method of manufacturing a sintered body according to a preferred embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

In a method of manufacturing a sintered body according to the present invention, a compact of a metal powder containing carbon is baked (sintered) under a predetermined atmosphere so as to produce a sintered body having a desired carbon content.

FIG. 1 is a flow chart showing a method of manufacturing a sintered body according to an embodiment of the present invention.

The method shown in FIG. 1 uses a metal powder to produce a sintered body. Specifically, as shown in FIG. 1, the manufacturing method of a sintered body includes a composition preparation process of preparing a composition containing a metal powder and an organic binder, a formation process of forming the composition into a primary compact, a removal process (degrease process/debinding process) of removing the organic binder from the primary compact so as to produce a secondary compact, and a baking process of baking (sintering) the secondary compact so as to produce a sintered body. These processes will be described in the order named.

1) Composition Preparation Process

First, a metal powder and an organic binder are prepared and kneaded by a kneader, so that a kneaded compound (composition) is produced. The metal powder is dispersed uniformly in the kneaded compound. It is desirable that the metal powder and the organic binder mixed in the kneaded compound should not react with each other or should have little reactivity with each other.

According to the present invention, a metal powder containing carbon (C) is used as the metal powder. The material of the metal powder is not limited to a specific one as long as it is a metal material containing carbon. Examples of the material of the metal powder includes stainless steel such as SUS-420 or SUS-440, carbon steel, die steel, high speed tool steel, various kinds of Fe-based alloy materials such as Fe—Ni—Co alloys or Fe—Ni alloys including Fe₂NiC or Fe₈NiC, low-carbon steel, various kinds of Ni-based alloy materials, and various kinds of Cu-based alloy materials.

Meanwhile, in the field of powder metallurgy, mechanical characteristics of a resultant sintered body have been improved by properly adjusting the composition of a metal powder. Specifically, it can be seen that mechanical characteristics such as tensile strength and hardness are improved in a sintered body having a given carbon (C) content (e.g., about 0.8 atm % to about 1.2 atm %).

In the conventional technology, however, even if a metal powder adjusted in carbon content in order that a sintered body has a desired carbon content is used to produce a sintered body, it is difficult to manufacture a sintered body having a desired carbon content for the reasons descried later.

In contrast to the conventional technology, the present invention demonstrates its effects and advantages effectively in a case where a metal powder including an Fe-based alloy material is used to produce a sintered body. Specifically, an Fe-based alloy sintered body having a desired carbon content can be produced efficiently by baking an Fe-based alloy powder containing carbon under a predetermined atmosphere. The resultant sintered body exhibits excellent mechanical characteristics.

It is desirable to use a metal powder having a carbon content in a range of about 0.05 wt % to about 2 wt %, preferably about 0.1 wt % to about 1.5 wt %. When a metal powder containing a small amount of carbon is used to produce a sintered body as described above, mechanical characteristics of the sintered body often vary according to a slight change of the carbon content. The present invention demonstrates its effects and advantages effectively in producing a sintered body in which the carbon content is required to be controlled with reliability.

The metal powder may contain two or more kinds of materials having different compositions. In this case, it is possible to produce a sintered body having an alloy composition that has not heretofore been produced by casting. Furthermore, it is easy to produce a sintered body having novel functions or many functions. Accordingly, it is possible to widen functions and applications of a sintered body.

The average particle diameter of the metal powder is not limited to a specific value. Nevertheless, it is desirable that the metal powder should have an average particle diameter of about 3 μm to about 30 μm, more preferably about 5 μm to about 20 μm. With use of the metal powder having an average particle diameter in the above range, it is possible to increase the flowability of a kneaded compound and thus obtain a kneaded compound having excellent formability (ease of formation). As a result, it is possible to enhance the density of a primary compact in the subsequent formation process and hence produce a sintered body having excellent mechanical characteristics as a final product.

Meanwhile, if the metal powder has a particle diameter as small as described above, the metal powder in a secondary compact described later is brought into contact with the atmosphere with an increased surface area. Accordingly, carbon tends to be desorbed in the subsequent baking process. The present invention demonstrates its effects and advantages effectively in a case where such a secondary compact is baked so as to produce a sintered body.

For example, such a metal powder can be manufactured by an atomization process (e.g., a water atomization method, a gas atomization method, or a high-speed spinning water atomization method), a reduction process, a carbonyl process, or a grinding process. It is desirable to use a metal powder manufactured by an atomization process. By an atomization process, it is possible to efficiently produce an extremely fine metal powder. When such a metal powder is employed as a material powder, a sintered body having a fine crystalline structure and excellent mechanical strength can be produced with reliability.

Furthermore, because each particle of a metal powder manufactured by an atomization process has a spherical shape close to a perfect sphere, a metal powder manufactured by an atomization process has excellent dispersibility and flowability. Accordingly, in the formation process, the kneaded compound can be filled into a forming die at a high filling fraction. Thus, a primary compact and a secondary compact having a fine complicated shape can readily be produced in the formation process and the removal process, which will be described later. However, when a secondary compact has a fine complicated shape, it tends to have a large surface area contacting the atmosphere. Therefore, carbon in the secondary compact is likely to be desorbed in the subsequent baking process because of reaction with an atmosphere gas. The present invention, however, demonstrates its effects and advantages effectively even in a case where a secondary compact having a complicated shape is baked so as to produce a sintered body.

Examples of the organic binder include various kinds of resins including polyolefine such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymer, acrylic resin such as polymethyl methacrylate and polybutyl methacrylate, styrene resin such as polystyrene, polyester such as polyvinyl chloride, polyvinylidene chloride, polyamide, polyethylene terephthalate, and polybutylene terephthalate, polyester, polyvinyl alcohol, and copolymer thereof, a variety of wax, paraffin, higher fatty acid (e.g., stearic acid), higher alcohol, higher fatty acid ester, and higher fatty acid amide. One or more materials of the above materials may be mixed in the organic binder.

Furthermore, it is desirable that the kneaded compound should have an organic binder content of about 2 wt % to about 40 wt %, more preferably about 5 wt % to about 30 wt %. With the kneaded compound having an organic binder content in the above range, it is possible to form a primary compact with high formability and enhance the density of the primary compact, so that the primary compact can have excellent stability in shape. Additionally, it is possible to reduce a difference in size between the primary compact and the secondary compact, i.e., a shrinkage percentage. As a result, the dimensional accuracy of the secondary compact and the sintered body can be improved.

A plasticizer may be added to the kneaded compound. Examples of the plasticizer include phthalate ester (e.g., DOP, DEP, and DBP), adipic acid ester, trimellitic acid ester, and sebacic acid ester. One or more materials of the above materials may be mixed in the plasticizer.

For example, in addition to the metal powder, the organic binder, and the plasticizer, various additives such as an antioxidant, a degrease accelerator, and a surface-active agent may be added to the kneaded compound as needed.

Kneading conditions vary depending upon various conditions including the metal composition or the particle diameter of a metal powder to be used, the composition of an organic binder, and the amounts of the metal powder and the organic binder to be mixed. For example, the kneading temperature is in a range of about 50° C. to about 200° C., and the kneading time is in a range of about 15 minutes to about 210 minutes.

Furthermore, the kneaded compound may be formed into pellets (small lumps). For example, each pellet has a particle diameter of about 1 mm to about 15 mm.

2) Formation Process

Next, the kneaded compound is formed into a primary compact having the same shape as a sintered body to be produced. A method of producing (forming) a primary compact is not limited to a specific one. For example, a primary compact may be formed by a metal injection molding (MIM) method or a compression molding method (compression powder molding method). Particularly, a metal injection molding method is suitable for formation of a primary compact.

The MIM method allows a relatively small primary compact or a primary compact having a fine complicated shape to be produced with a near net shape (a shape close to a final product). Thus, the MIM method can advantageously make full use of characteristics of the metal powder to be used. Although the MIM method can form a fine complicated shape, it is likely to increase a surface area of the secondary compact contacting the atmosphere. Accordingly, the aforementioned tendency becomes more significant. The present invention, however, demonstrates its effects and advantages effectively even in a case where a secondary compact produced by an MIM method is baked so as to produce a sintered body.

A method of producing a primary compact will be described below with use of a typical example of an MIM method.

First, injection molding is performed on the kneaded compound produced in the process 1) or pellets granulated from the kneaded compound by an injection molding machine. As a result, a primary compact having a desired shape and size is produced. In this case, by properly selecting a forming die, it is possible to produce a primary compact having a complicated shape with ease.

In the primary compact thus produced, the metal powder is substantially dispersed uniformly in the organic binder.

The shape and size of the primary compact to be produced are determined in expectation of a shrinkage of the primary compact due to subsequent degrease and sintering.

Forming conditions in injection molding vary depending upon various conditions including the composition or the particle diameter of a metal powder to be used, the composition of an organic binder, and the amounts of the metal powder and the organic binder to be mixed. For example, the material temperature is preferably in a range of about 80° C. to about 200° C., and the injection pressure is preferably in a range of about 2 MPa to about 30 MPa (about 20 kgf/cm² to about 300 kgf/cm²).

3) Removal Process (Degrease Process)

A degrease process (binder removal process) is performed on the primary compact produced in the process 2), so that a secondary compact (degreased compact) is produced. For this degrease process, heat treatment is performed under an atmosphere including an oxidizing gas such as air or oxygen, a reducing gas such as hydrogen or carbon monoxide, an inert gas such as nitrogen, helium, or argon, or a mixed gas containing one or more of these gases, or under an reduced-pressure atmosphere. In this case, conditions for heat treatment vary to some extent, for example, depending upon the temperature at which the organic binder begins to be decomposed. The heat treatment is preferably performed at a temperature of about 100° C. to about 750° C. for about 0.5 hour to about 40 hours, more preferably at a temperature of about 150° C. to about 600° C. for about 1 hour to about 24 hours.

The degrease process with heat treatment may be performed by a plurality of steps (stages) for various purposes (e.g., for the purpose of shortening the degrease time). For example, a first half of the degrease process may be performed at a low temperature while a second half of the degrease process may be performed at a high temperature. Alternatively, a low-temperature degrease step and a high-temperature degrease step may be repeated.

Furthermore, the degrease process may be performed by eluting a specific component in the organic binder or the additives with use of a predetermined solvent (a fluid such as a liquid or a gas).

Thus, the organic binder is removed. As a result, a secondary compact is produced.

The organic binder may not be removed completely by the degrease process. For example, a portion of the organic binder may be left in the compact at the time of completion of the degrease process.

In this manner, it is possible to produce a secondary compact having excellent capability of maintaining its shape (form).

In the present embodiment, the processes 1) to 3) constitute a first process of forming a metal powder containing carbon into a compact having a predetermined shape.

For example, the aforementioned metal powder may be pressed into a predetermined shape so as to provide a compressed compact having a desired shape. Such a compressed compact may be used instead of the secondary compact produced in the process 3). In this case, the pressing process constitutes a first process of forming a metal powder containing carbon into a compact having a predetermined shape.

4) Baking Process

The secondary compact produced in the process 3) is baked in a baking furnace or the like. Thus, the secondary compact is sintered, so that a sintered body is produced (second process). By the sintering, the metal powder is dispersed at interfaces between particles so as to cause grain growth, thereby forming a crystalline structure. As a result, it is possible to produce a closely packed sintered body having a high density.

The baking temperatures vary to some extent depending upon the composition of the metal powder and the like. For example, the baking temperature is preferably in a range of about 1,000° C. to about 1,400° C., more preferably about 1,100° C. to about 1,300° C. Under the sintering temperature in the above range, the dispersion and grain growth of the metal powder are optimized to thereby provide a sintered body having excellent characteristics such as mechanical strength, dimensional accuracy, and external appearance. The sintering temperature in the sintering process may vary (increase or decrease) with time in or out of the above range.

The sintering period is preferably in a range of about 0.5 hour to about 7 hours, more preferably about 1 hour to about 4 hours.

Meanwhile, in the present invention, the metal powder contains carbon at a given percentage. With the conventional technology to produce a sintered body from such a metal powder, a secondary compact is baked in a baking furnace under an a non-oxidizing atmosphere such as an inert gas atmosphere or a reducing gas atmosphere, an air atmosphere, or a reduced-pressure atmosphere having a high vacuum (e.g., a pressure of 13 Pa (0.1 Torr) or less), thereby producing a sintered body.

If a secondary compact is baked under various gas atmospheres such as a non-oxidizing atmosphere and an air atmosphere, then carbon in the secondary compact reacts with H₂ (hydrogen gas) or O₂ (oxygen gas) contained in the atmosphere. As a result, the carbon in the secondary compact is problematically desorbed from the secondary compact. Specifically, when carbon in the secondary compact reacts with H₂, it is converted into a hydrocarbon gas such as CH₄ and desorbed from the secondary compact. Furthermore, when carbon in the secondary compact reacts with O₂, it is converted into CO or CO₂ and desorbed from the secondary compact.

These reactions decrease the carbon content of the secondary compact so that the carbon content of a sintered body as a final product becomes lower than a desired carbon content. Thus, the mechanical characteristics of the sintered body are deteriorated. As described above, such a problem becomes more significant when the metal powder has a low carbon content.

Additionally, because a decrease of the carbon content proceeds when the secondary compact is brought into contact with H₂ or O₂ contained in the atmosphere, it is likely to depend upon a shape of the secondary compact. That is, the carbon content tends to be lowered in a secondary compact having a complicated shape with a large surface area. Accordingly, the degree of the decrease of the carbon content varies depending upon the shape of a secondary compact. This problem causes variations in mechanical characteristics between a plurality of sintered bodies having different shapes or between portions having different shapes in a sintered body.

On the other hand, if a secondary compact is baked under a reduced-pressure atmosphere having a high vacuum, then a pressure in a baking furnace is lowered. Furthermore, O₂ is produced by decomposition (dissociation) of components of the baking furnace or support members for supporting the secondary compact or by dissociation of moisture adsorbed in these components or members. Accordingly, the same problem as described above arises. Moreover, large and expensive facilities are required for a vacuum pump or a pressure vessel in order to maintain a high vacuum. Therefore, manufacturing cost is unavoidably increased. Additionally, a pressure vessel capable of maintaining a high vacuum has limitation in volume. Accordingly, such a pressure vessel can accommodate only a limited number of secondary compacts. As a result, the production efficiency is problematically low.

According to the present invention, a secondary compact is placed in a hermetically sealed space having an atmosphere in which a pressure is adjusted to 60 kPa to 140 kPa (450 Torr to 1,050 Torr). Further, the sum of partial pressures of H₂ (hydrogen gas) and O₂ (oxygen gas) in the atmosphere is adjusted to a value not more than 3 Pa. Then the secondary compact is baked in this hermetically sealed space. With this method, it is possible to efficiently produce a sintered body containing carbon at a desired percentage.

Furthermore, since the carbon content is prevented or inhibited from being lowered, a variation in decrease of the carbon content can be reduced. Therefore, it is possible to produce a sintered body having desired mechanical characteristics.

According to the present invention, the sum of a partial pressure of H₂ and a partial pressure of O₂ in the atmosphere is set to be at most 3 Pa. Therefore, the amounts of H₂ and O₂ to react with carbon in a secondary compact can be reduced. Thus, carbon is prevented from being consumed to a large extent.

In a case where an atmosphere gas is continuously supplied into a conventional continuous baking furnace, since the atmosphere gas unavoidably contains H₂ gas and O₂ gas as impurities, the H₂ gas and O₂ gas are also supplied (replenished) constantly to the baking furnace. Accordingly, even if a gas having a low partial pressure of H₂ and a low partial pressure of O₂ is used as the atmosphere gas, a decrease of the carbon content of the secondary compact is unavoidably caused.

In contrast to the conventional method, according to the present invention, a secondary compact is placed in a hermetically sealed space. Therefore, once H₂ and O₂ contained in the atmosphere are consumed by reactions with carbon in the secondary compact, further consumption of carbon in the secondary compact can be prevented. Thus, the secondary compact can be baked while carbon in the secondary compact can be prevented or inhibited from being reduced. Hence, it is possible to produce a sintered body having a desired carbon content with ease.

According to the present invention, the sum of a partial pressure of H₂ and a partial pressure of O₂ in the atmosphere should not be more than 3 Pa. It is desirable that the sum of the partial pressures should be at most 2.5 Pa, more preferably at most 1.5 Pa. With this configuration, it is possible to further reduce consumption of carbon in the secondary compact and make the carbon content of a sintered body closer to a desired value. As a result, it is possible to produce a sintered body having better mechanical characteristics.

The secondary compact is placed in a hermetically sealed space within a closed vessel or the like. The pressure of the hermetically sealed space is adjusted to 60 kPa to 140 kPa (450 Torr to 1,050 Torr). This pressure can be maintained sufficiently with a simple closed vessel because of a small difference from an atmospheric pressure.

Furthermore, it is possible to shorten a period of time required to increase or decrease the pressure in the closed vessel. As a result, a production efficiency of a sintered body can be enhanced.

The pressure of the hermetically sealed space should be in a range of 60 kPa to 140 kPa (450 Torr to 1,050 Torr). It is desirable that the pressure of the hermetically sealed space should be in a range of about 80 kPa to about 120 kPa (about 600 Torr to about 900 Torr). A simpler closed vessel can be used within this range. Furthermore, when the pressure of the hermetically sealed space is in the above range, a pressure difference between the interior and exterior of the hermetically sealed space can be made extremely small. Accordingly, the closed vessel does not need to be equipped with a special pressure proof mechanism, and cost of the baking process can be reduced.

The atmosphere in the closed vessel may contain any gas components. Nevertheless, it is desirable that the atmosphere primarily should contain an inert gas such as nitrogen, helium, or argon. An inert gas is most unlikely to react with carbon or other elements in the secondary compact. Therefore, it is possible to prevent the composition in the secondary compact from unexpectedly varying in the baking process.

Particularly, it is desirable to use an argon gas as the inert gas. Since an argon gas is a noble gas, it has a low reactivity with most of elements. Therefore, it is possible to more reliably prevent the composition in the secondary compact from unexpectedly varying in the baking process. Furthermore, since an argon gas can be obtained at a relatively low cost among noble gases, it can suitably be used as the atmosphere gas.

For example, the aforementioned baking process can be performed by a baking furnace (closed vessel) as shown in FIG. 2.

FIG. 2 is a vertical cross-sectional view schematically showing a baking furnace (closed vessel) 10 used in a method of manufacturing a sintered body according to the present invention. In the following description, the upper and lower sides in FIG. 2 will be referred to as “upper” and “lower,” respectively.

As shown in FIG. 2, the baking furnace 10 includes a furnace body 11 having an opening portion 12 formed in a side surface thereof, a cover 20 capable of closing the opening portion 12 and hermetically sealing the furnace body 11, and a stage 14 provided in an internal furnace space 13 within the furnace body 11 for supporting secondary compacts 30 thereon.

Furthermore, the baking furnace 10 also includes an gas supply valve 15 provided at an upper portion of the furnace body 11, a gas supply device 40, and a pipe 16 connecting the gas supply valve 15 and the gas supply device 40 to each other. Thus, the baking furnace 10 is configured to supply an atmosphere gas from the gas supply device 40 into the internal furnace space 13 or stop the supply of the atmosphere gas via the pipe 16 and the gas supply valve 15.

The baking furnace 10 has a gas discharge valve 17 provided at a lower portion of the furnace body 11. Thus, the baking furnace 10 is configured to discharge the atmosphere gas in the internal furnace space 13 to the exterior of the furnace body 11 or stop the discharge of the atmosphere gas via the gas discharge valve 17.

In the internal furnace space 13, a heater 18 is provided along a wall surface of the furnace body 11. The heater 18 is connected to a power source device (not shown) via wiring. When the heater 18 is supplied with electric power, it generates heat so as to heat the atmosphere gas and the secondary compacts 30 in the internal furnace space 13.

Baking operation of the secondary compacts 30 in the baking furnace 10 will be described below.

First, the secondary compacts 30 are placed on the stage 14. Then the opening portion 12 is closed by the cover 20. Subsequently, the gas discharge valve 17 and the gas supply valve 15 are opened. Thus, the internal furnace space 13 is filled with an atmosphere gas that meets the aforementioned conditions. Before the supply of the atmosphere gas, the pressure in the internal furnace space 13 may be reduced by a vacuum pump or the like as needed. In such a case, it is possible to enhance the purity of the atmosphere gas in the internal furnace space 13.

Thereafter, the gas discharge valve 17 and the gas supply valve 15 are closed so that the internal furnace space 13 is hermetically sealed.

Next, electric power is supplied to the heater 18 so that the secondary compacts 30 are baked at the aforementioned baking temperature for the aforementioned baking time. In this manner, a sintered body is produced.

The sintered body thus produced has a desired carbon content and demonstrates excellent mechanical characteristics.

Although a preferred embodiment of a method of manufacturing a sintered body according to the present invention has been shown and described in detail, it should be understood that the present invention is not limited to the illustrated embodiment. For example, any additional process may be added to a method of manufacturing a sintered body as desired. Furthermore, the atmosphere in the closed vessel may be changed during the baking process as needed. The baking process may be performed subsequently to the removal process (degrease process).

EXAMPLES

1. Manufacturing Sintered Bodies

i) First, a powder of stainless steel SUS-440C produced by a water atomization method (PF-20F made by EPSON ATMIX Corporation) was prepared. This powder had an average particle diameter of 10 μm. A mixture of polypropylene and wax (organic binder) was also prepared. The powder of stainless steel and the mixture of polypropylene and wax were weighed with a weight ratio of 9:1 and mixed with each other. Thus, a mixed material was obtained.

ii) Then the mixed material was kneaded by a kneader, so that a kneaded compound was produced.

iii) Next, injection molding was performed on the kneaded compound under the following conditions by an injection molding machine so as to produce primary compacts. At that time, primary compacts having three types of shapes (Shape A, Shape B, and Shape C) with different surface areas were produced. Ten primary compacts were produced for each shape type. Accordingly, 30 primary compacts were produced in total.

(Forming Conditions)

-   -   Material temperature: 150° C.     -   Injection pressure: 11 MPa (110 kgf/cm²)

iv) Subsequently, heat treatment (degrease process) was performed on the produced primary compacts under the following conditions so as to produce secondary compacts (degreased compacts).

(Degrease Conditions)

-   -   Heating temperature: 500° C.     -   Heating time: 2 hours     -   Heating atmosphere: nitrogen gas

v) Then the produced secondary compacts were baked under the following baking conditions. Thus, sintered bodies were produced (Example I). The baked process was performed in a hermetically sealed state in which the secondary compacts were housed in a baking furnace capable of hermetically sealing the interior of the furnace.

(Baking Conditions)

-   -   Baking temperature: 1,235° C.     -   Baking time: 6 hours     -   Heating atmosphere: argon gas (The sum of H₂ partial pressure         and O₂ partial pressure was 2.0 Pa.)     -   Pressure of atmosphere: atmospheric pressure (100 kPa)

Other sintered bodies were produced in the same manner as in Example I except that the pressure of the heating atmosphere was changed into 133 kPa (1,000 Torr) in Step v) (Example II). The sum of H₂ partial pressure and O₂ partial pressure was 2.7 Pa.

Still other sintered bodies were produced in the same manner as in Example I except that the pressure of the heating atmosphere was changed into 67 kPa (500 Torr) in Step v) (Example III). The sum of H₂ partial pressure and O₂ partial pressure was 1.3 Pa.

Other sintered bodies were produced in the same manner as in Example I except that the sum of H₂ partial pressure and O₂ partial pressure in the heating atmosphere was set to be 0.5 Pa in Step v) (Example IV).

Still other sintered bodies were produced in the same manner as in Example I except that the baking process was performed with a baking furnace capable of continuously supplying an atmosphere gas in Step v) (Comparative Example V).

Other sintered bodies were produced in the same manner as in Example I except that the sum of H₂ partial pressure and O₂ partial pressure in the heating atmosphere was set to be 20 Pa in Step v) (Comparative Example VI).

II. Evaluating the Sintered Bodies

Composition analysis was performed on the sintered bodies obtained in Examples I to IV and Comparative Examples V and VI with an electron probe microanalyzer (EPMA).

First, composition analysis was performed on the powder of SUS-440C used in Examples I to IV and Comparative Examples V and VI. Then composition analysis was performed on the sintered bodies obtained in Examples I to IV and Comparative Examples V and VI. The carbon content of the powder of SUS-440C and the carbon content of each sintered body were measured. The carbon contents of the sintered bodies of Examples I to IV and Comparative Examples V and VI were represented by a relative value on the assumption that the carbon content in the powder of SUS-440C was defined by 1.

Next, the carbon contents of the thirty sintered bodies obtained in each of Examples I to IV and Comparative Examples V and VI were compared. Specifically, a difference between a maximum value and a minimum value of the carbon contents of the thirty sintered bodies was calculated as a range of the carbon contents. The ranges of the carbon contents of the sintered bodies were compared and evaluated. Table 1 shows results of the evaluation.

TABLE 1 Baking Evaluation conditions results Partial Range of pressure Carbon carbon of content contents H₂ + O₂ Atmosphere (Relative (relative [Pa] gas value) values) Reference example — — 1 — (Metal powder) Example I 2.0 Sealed 0.37 0.04 (closed) Example II 2.7 Sealed 0.35 0.05 (closed) Example III 1.3 Sealed 0.41 0.02 (closed) Example IV 0.5 Sealed 0.40 0.02 (closed) Comparative 2.0 Continuous 0.19 0.13 Example V supply Comparative 20 Sealed 0.22 0.15 Example VI (closed)

As shown in Table 1, average values of the carbon contents of the sintered bodies in Examples I to IV ranged from 0.35 to 0.41 as relative values in the assumption that the carbon content of the powder of SUS-440C was defined by 1. In contrast to Examples I to IV, average values of the carbon contents of the sintered bodies in Comparative Examples V and VI ranged from 0.19 to 0.22. Thus, it can be seen that desorption of carbon could be reduced in the baking process of Examples I to IV as compared to Comparative Examples V and VI.

Furthermore, when comparing ranges (or variations) of the carbon contents of the sintered bodies obtained in Examples I to IV and Comparative Examples V and VI, the ranges of the carbon contents of the sintered bodies of Examples I to IV were much narrower (smaller) than those of Comparative Examples V and VI. This result shows that the present invention can efficiently produce sintered bodies having a desired carbon content irrespective of their shapes even if those sintered bodies have different shapes (surface areas). Additionally, as can be seen from the results, since the range of the carbon contents of sintered bodies can be made small, the carbon contents of the sintered bodies can be made close to a desired value.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims. 

1. A method of manufacturing a sintered body, the method comprising: forming a metal powder containing carbon into a compact having a predetermined shape; and baking the compact in a hermetically sealed space so as to produce a sintered body, the hermetically sealed space having an atmosphere having a pressure of 60 kPa to 140 kPa and containing a hydrogen gas and an oxygen gas, a sum of partial pressures of the hydrogen gas and the oxygen gas being not more than 3 Pa.
 2. The method as recited in claim 1, wherein the atmosphere of the hermetically sealed space primarily contains an inert gas.
 3. The method as recited in claim 2, wherein the inert gas comprises an argon gas.
 4. The method as recited in claim 1, wherein the metal powder has an average particle diameter of 3 μm to 30 μm.
 5. The method as recited in claim 1, wherein the metal powder has a carbon content of 0.05 atm % to 2 atm %.
 6. The method as recited in claim 1, wherein the metal powder is made of an Fe-based alloy material.
 7. The method as recited in claim 1, wherein the metal powder forming process comprises: forming a composition containing the metal powder and a binder into a predetermined shape so as to produce a primary compact; and removing the binder from the primary compact so as to produce a secondary compact as the compact.
 8. The method as recited in claim 7, wherein the forming process of the composition comprises metal injection molding of the composition to produce the primary compact.
 9. A sintered body manufactured by the method as recited in claim
 1. 